The invention relates to a method for marine seismic exploration, in which an array of seismic sources is used, in particular an air gun array comprising a plurality of air guns. The seismic method is a known geophysical exploration method to investigate the character of subsurface earth formations and more particularly to survey subterranean formations.
In seismic surveys seismic waves are generated periodically at preselected locations of the earth's surface and various characteristics of the waves, as they enter the earth and are reflected or refracted from subsurface earth strata, are measured.
The waves are detected by sensitive instruments, placed at varying distances from the preselected locations and are subsequently converted into electrical signals which may be further processed to derive data. The data may be plotted to obtain a picture or map of the investigated area, which may indicate the presence, if any, of structural traps capable of holding hydrocarbons like oil and/or gas.
The amplitude of the seismic waves of interest is a function of the size of the source and it will be clear that a wave having a relatively high amplitude may be studied easier than a wave having a lesser amplitude. A well known method of generating seismic waves is detonation of one or more charges of dynamite or other suitable explosive, embedded at predetermined locations in the earth.
However, marine seismic exploration is complicated by certain problems, which either do not exist or are relatively unimportant in land seismic exploration.
For example, it will be clear that detonation of a charge of dynamite in a marine environment may cause destruction to marine life.
To avoid such problem it has now become a usual practice in marine seismic exploration to use non-dynamite seismic sources, for example air guns. The air guns are towed behind a marine vessel by means of a towing cable and the air guns may suddenly release confined volumes of high pressure gas. This produces acoustical pressure pulses having a certain acoustic energy, that are converted to seismic waves as they enter the earth. The acoustic signal, generated by an air gun, shows a series of pressure pulses resulting in a corresponding oscillatory frequency spectrum having a fundamental frequency which is related to the amount and pressure of released gas volume. Moreover, air guns ae low intensity energy sources, so the acoustic energy waves produced by an air gun will have low amplitude and any electrical signal derived therefrom will have a relatively low signal-to-noise ratio. The signal strength produced by an air gun is several orders of magnitude less than that generated by a dynamite explosion or using other suitable explosives. These features make a single air gun less effective as a seismic source.
Moreover, marine seismic exploration technology requires a way of detecting seismic waves, which involves added noise. For example, when submerged areas are explored, the detectors cannot normally be fixedly located so as to detect a number of sources of equal significance.
Instead, it is conventional in marine seismic exploration to locate the detectors in a continuously towed streamer cable, which extends from the rear of a towing vessel. The acoustic waves reflected from subsurface formations are detected, while the vessel is proceeding along a prescribed course over the area of interest. However, such a procedure produces towing noise that lowers the quality of the signals to be obtained.
To improve both the signal strength and the signal shape it has been proposed to use a plurality of seismic sources disposed in an array, in particular an array of air guns. It has been found advantageous in marine seismic exploration to generate a plurality of acoustic waves from such an array in order to provide a composite acoustic wave of satisfactory amplitude and frequency content. Air guns having various volume capacities are generally used in such arrays in order to produce a composite acoustic wave having a broad frequency band, as air guns of various volumes generate acoustic waves with different frequency spectrum and corresponding phase spectrum. The frequency content and in particular the fundamental frequency of the generated acoustic wave is dependent upon the volume capacity of the air gun.
To provide a full spectrum of high energy acoustic pulses it is desirable to generate a number of acoustic waves of various fundamental frequencies. Several arrays have been designed and implemented using an increasing number of guns and an increasing amount of compressed air power. The source strength has been gradually increased and adequate levels are now available.
However, problems associated with noise appearance and loss of useful acoustic wave energy in water still exist and are not solved satisfactorily by the prior art. For example, meaningful information contained by seismic waves directly reflected from the ocean bottom is often masked due to the reception of horizontally travelling noise caused by reflections of horizontal seismic waves from underwater obstacles and the like. The high frequency radiation in more horizontal directions should be suppressed to avoid water trapped noise to dominate the seismic record. However, after this it might well be that still a significant amount of additional energy is needed to overcome the streamer noise and to compensate for non-elastic losses.
Further, present day air gun arrays are designed to emit shot signals to improve the spectrum of the signal and its determination. This may be achieved by cancelling the "tail energy" of the signal which is partly a destructive exercise and physically means a loss of possible useful energy.